Relationship between changes in length and force in in vitro reconstructed anterior cruciate ligament

The effect of tibial and femoral attachment site on the length change and force of an anterior cruciate ligament graft during unloaded flexion in eight cadaver specimens was examined. Two tibial sites (anteromedial and central portion of the anterior cruciate ligament attachment) and three femoral sites (anterior and central portions of the anterior cruciate ligament attachment, and over-the-top) were evaluated. Graft length changes between all combinations of attachment sites were measured from full extension to 150 degrees of passive flexion at 15 degrees intervals using the displacement of a 2-mm inextensible cord. The anterior cruciate ligament was then reconstructed using a Kennedy Ligament Augmentation Device, and graft forces at the same angles of passive flexion were measured with a buckle transducer. Graft length change and force were more affected by the femoral attachment site than the tibial site. There was a close correlation between length change and force measurements in flexion, but not near extension. The pattern of force and length change versus flexion angle for a given combination of attachment sites sometimes varied over the knees tested. Our results suggest that intraoperative isometry measurements are worthwhile for indicating an overloaded graft in flexion; however, length changes near extension may not adequately reflect graft force, creating the possibility that a graft may be more highly loaded than realized.